P0198-25-001_12_Gympie_Street_Tewantin_SWMP

Required Mitigation – Installation of a Detention Basin

Due to the projected increase in post-development peak flows, mitigation measures are required to ensure
compliance with Noosa Shire Council stormwater management requirements and to prevent potential
downstream impacts. The most suitable solution is the installation of a detention basin, designed to achieve the
following outcomes:

     •    Attenuate peak discharge by temporarily storing excess runoff and releasing it at controlled, pre-
          development rates

     •    Control flow velocities, thereby reducing the risk of downstream erosion and infrastructure overloading
          and

     •    Improve water quality by facilitating the settlement of suspended solids and associated pollutants prior
          to discharge.

The detention basin will be designed to ensure that post-development peak flows do not exceed pre-
development flow conditions for all relevant design storm events. Further hydrological modelling will be
undertaken to appropriately size the basin and confirm its effectiveness in mitigating increased flow volumes and
velocities.


3.4         Stormwater Detention and Mitigation Strategy
A bioretention basin is proposed on the north-eastern side of the development, as shown in Civil set of plans for
consideration. The basin has been designed to retain and control stormwater flows so that post-development peak
discharges remain at or below pre-development levels. The design provides attenuation across all assessed AEP
storm events, consistent with the guidelines of the QUDM. Refer to Table 3-2 for basin design parameters.

As outlined in preceding sections, most of the site’s natural drainage flows northward. Under ideal conditions, the
preferred location for the bioretention basin would be along the northern boundary, enabling full capture of the
developed catchment and discharge to the site’s natural overland flow path. However, a sewer easement occupies
this boundary, precluding the installation of major stormwater infrastructure. Consequently, the basin has been
positioned in the north-eastern sector, enabling discharge to Council’s dedicated reserve to the north of the Butler
Street Cul de sac.

Flows have been modelled to assuming the entire site drains to the northeast, into the Butler Street road reserve.
This modelling approach provides a clear and consistent comparison between pre- and post-development
scenarios and demonstrates anticipated improvements to local drainage conditions.
Table 3-2: Detention Basin parameters

 Parameter                                      Value

 1% AEP Storage Volume                          289 m³

 1% AEP Water Depth                             0.8 m

 Lower Discharge Control                        150 mm orifice at 2.65 m AHD

 Secondary Discharge Control                    100 mm orifice at 2.85 m AHD

 Third Discharge Control                        600 x 600 mm field inlet at ODD at 3.05 m AHD

 Wier Level                                     3.45 m AHD

 Basin Excavated Level                          1.65 m

 Top of Filter Media Level                      2.35 m

 Top of Retaining Wall Level                    3.65 m

 Outlet Pipe Size                               275 mm orifice into 375 mm pipe

 Outlet Pipe Invert Level                       1.710 m

 12 GYMPIE STREET, TEWANTIN
 Stormwater Management Plan                                                                                     15
 Client Reference: P0198-25-001          4 November 2025
 Prepared for Novato Group